Is a new blockchain really needed to do this? Why not just use regular bitcoins to pay for computing power? Also, don't most calculations worth paying for involve sensitive data that you wouldn't want available to the public?

I like the idea of giving the network/coin real value (computing power for hire) and using the immense power of the network for something productive. I heard some suggestions to limit mining power per IP adres to 5 MH/s to limit the waste of electricity and the remaining capacity could then be used for usefull purposes... I'm not quite sure how it would work but I think you've got an interesting starting point for discussion

Greencoin (ZC):...(1) - I think the current VOTING in Bitcoin system (with the current "difficulty"-system) can be a good compromise - but the voting must be scheduled at a low and fixed frequence with a new communication protocol. ...

Satoshi' paper (only the first paragraph "Abstract") criticism:..."Digital signatures provide part of the solution, but the main benefits are lost if a trusted third party is still required to prevent double-spending."

- A trusted third party (in greencoin) isn't required for prevent double-spending, it is required to prevent 50%+1 attack. The trusted third party should be the community of users/miners/guests/moderators of greencoin... The community of our money.

"The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of CPU power."

- The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of trusted miners. "power" is a possible means of a "centralised authority" - please, do not use it for prevent double-spending

"The network itself requires minimal structure. Messages are broadcast on a best effort basis, and nodes can leave and rejoin the network at will, accepting the longest proof-of-work chain as proof of what happened while they were gone."

Greencoin (ZC):...(1) - I think the current VOTING in Bitcoin system (with the current "difficulty"-system) can be a good compromise - but the voting must be scheduled at a low and fixed frequence with a new communication protocol. ...

Satoshi' paper (only the first paragraph "Abstract") criticism:..."Digital signatures provide part of the solution, but the main benefits are lost if a trusted third party is still required to prevent double-spending."

- A trusted third party (in greencoin) isn't required for prevent double-spending, it is required to prevent 50%+1 attack. The trusted third party should be the community of users/miners/guests/moderators of greencoin... The community of our money.

"The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of CPU power."

- The longest chain not only serves as proof of the sequence of events witnessed, but proof that it came from the largest pool of trusted miners. "power" is a possible means of a "centralised authority" - please, do not use it for prevent double-spending

"The network itself requires minimal structure. Messages are broadcast on a best effort basis, and nodes can leave and rejoin the network at will, accepting the longest proof-of-work chain as proof of what happened while they were gone."